HYOMANDIBULA OF THE GNATHOSTOME FISHES 607 
of embryos of Ceratodus, and that the later development of car- 
tilage said by Edgeworth to spread round and enclose the ner- 
vus hyomandibularis facialis is simply the chondrification, in situ, 
of the extrabranchial of the arch, and its concomitant fusion 
with the pharyngohyal. The anterior articular head of the tele- 
ostean hyomandibula is then, as are probably the corresponding 
cartilages in Torpedo and Ceratodus, an interarcual cartilage 
which has fused with the pharyngeal element of the hyal arch, 
and the posterior articular head is the extrabranchial of the arch 
fused with the same element; and, because of the much more 
advantageous articulation thus acquired, the primitive dorsal end 
of the pharyngeal element has lost, or never acquired, direct 
contact with the neurocranium. The interhyal (stylohyal) so 
closely resembles the epihyal of Ceratodus and the Batoidei that 
it is quite unquestionably that element, and this is in accord with 
Stéhr’s (82) statement that it is segmented from the dorsal 
end of the ceratohyal after that element has separated from the 
hyomandibula, exactly as it is said by Gegenbaur to be seg- 
mented from that element in the Batoidei. 
This thus accounts, in the Teleostei, for all the independent 
cartilaginous elements that take part in the formation of this 
dorsal half of the inner cartilaginous bar of the hyal arch in the 
Elasmobranchii and Dipneusti, and, if the symplectic be not 
simply a ventral process of the pharyngohyal, similar to the one 
so named by Gegenbaur in the Batoidei, it must be looked for 
in other cartilages of the region, and it would seem as if it must 
be a specially modified branchial ray of the mandibular arch. 
That it is an entirely new element here added to the hyal arch 
seems wholly improbable. 
When the hyomandibular visceral cleft began to undergo re- 
duction it is probable that it was first pinched off and closed at 
the middle of its length, for remnants or rudiments of the bran- 
chiae of the cleft have persisted both dorsal and ventral to this 
point—dorsally as the spiracular branchiae and ventrally as the 
thyreoid. It is also natural that the cleft should here have 
been first closed, for, as is actually the case in the branchial 
arches of living fishes, the dorsal and ventral ends of the inner 
